With the development of membrane science and technology and the innovation of water treatment technologies in recent years, membrane separation technology and membrane separation devices based on membrane separation technology have been widely adopted in the field of water treatment. As their applications become more popular and manufacturing technologies mature, the price of membrane separation devices have dropped. In general, however, one-time investment and operating costs for membrane-based water treatment technology are still considerably higher than conventional water treatment technologies. Particularly, due to their short lifespan, the replacement cycle of membrane separation devices is short, resulting in high depreciation expense of industrial applications of membrane-based water treatment, which largely restricts the application range of membrane separation technology in the field of water treatment.
Nowadays, membrane fouling is considered in the art as a major cause of the short lifespan of membrane separation devices. Membrane fouling is defined as reversible or irreversible deposition (including adsorption, clogging, sedimentation and filter cake formation) of substances such as particles, gels, emulsions, suspensions, large molecules and salts on a membrane surface or in a membrane pore. Microscopic details of membrane fouling are complex, especially when the membrane separation device is used to separate biomass and water in a wastewater treatment system, e.g., Membrane Bioreactor (MBR). At present, it is believed by the theorists in the art that the causes of membrane fouling include concentration polarization, pore clogging, gel layer, cake layer and mineral scaling. Concentration polarization and pore clogging generally occur several seconds or minutes after filtration starts. The gel layer forms gradually several minutes or hours afterwards, but its further thickening is slow. Therefore, in the early stage of membrane separation, filtering resistance increases rapidly, but enters a relatively slowly increasing phase afterwards, which mainly includes membrane fouling that results from cake layer formation and mineral scaling. The development of mineral scaling is slow; however, if the cake layer thickens rapidly, the filtering resistance will greatly increase again, which may even make the membrane to lose its water producing capability. Substances that may cause membrane fouling can be classified as: (1) particles, e.g., suspensions in raw water and floccules (consisting of microorganisms) in a membrane bioreactor, which generally cause cake layer formation; (2) large molecules of soluble organic compounds, e.g., soluble organic compounds in raw water, Soluble Microbial Products (SMPs) and Extra-Cellular Polymers (ECPs) accumulated in MBRs and some microorganisms, which generally cause pore clogging and gel layer formation; and (3) inorganic compounds, e.g., carbonates and sulfates in raw water, which generally cause mineral scaling.
Membrane fouling is inevitable in use of membrane separation devices. Therefore, while conducting studies on its formation mechanism and affecting factors, researchers are also seeking for an economic and effective solution for preventing and removing membrane fouling. At present in practice, when membrane fouling develops to a certain extent, an online or offline cleaning measure is to be taken on the membrane separation device to recover filtering performance of the membrane. The cleaning measure can be classified as physical methods and chemical methods. The physical methods include forward hydraulic cleaning which uses a gas, water or gas-water mixture to wash the normal working surface of the membrane, reverse hydraulic cleaning which uses a gas, water or gas-water mixture to permeate the membrane in a direction opposite to the normal working filtering direction of the membrane, and cleaning of the membrane based on ultrasonic waves, etc. The chemical methods mainly include forward chemical cleaning which uses a cleaning solution containing a chemical agent at a certain concentration to soak the normal working surface of the membrane, and reverse chemical cleaning which permeates the membrane in a direction opposite to the normal working filtering direction of the membrane. More information regarding the methods based on forward and reverse hydraulic cleaning can be found in Chinese patents and patent applications 95194986.1, 98125099.8, 02205772.2, 02224060.8, 200320110568.6, 200510013249.7 and 200580013230.0; more information regarding the methods based on reverse chemical cleaning can be found in Chinese patent 200510115862.X. And Chinese patent applications 200580046369.5 and 200610011310.9 provide a method which combines forward and reverse hydraulic cleaning and chemical cleaning.
Forward hydraulic cleaning prevents deposition of particles on the membrane surface mainly by a hydraulic shear force resulting from cross flow of a gas, water or gas-water mixture on the normal working surface of the membrane, which may control membrane fouling caused by the cake layer to a certain extent. But it has no significant effect in controlling the development of the gel layer due to deposition of gel substances on the membrane surface, and has no effect at all in controlling pore clogging due to gels and small molecules. Consequently, using solely forward hydraulic cleaning can not maintain a good cleaning effect in a long term. Moreover, in order to enhance the effect of forward hydraulic cleaning and to keep the water treatment plant's production process from being affected by membrane cleaning, the membrane separation device generally operates intermittently, with inoperative time being approximately 20% of the total time. The cross flow of the gas, water or gas-water mixture is continuous, hence, when the membrane separation device halts, the force that attaches the particles to the membrane surface disappears instantaneously, and some of the deposited particles may return to the liquid-phase body. This method is called “keeping aerating while no sucking” and has been widely adopted in MBRs that obtain the membrane permeating liquid in a negative pressure based manner. Rapid development of membrane fouling can be controlled to a certain extent, however, this cleaning effect is limited and is achieved with increased power consumption for speeding up the cross flow and reduced operating time of the membrane, which result in increased operating costs of the system, more membrane separation devices to be configured, and increased construction costs of the system.
The essential of reverse hydraulic cleaning and reverse chemical cleaning is to use a gas, cleaning water or cleaning solution containing a certain chemical agent to permeate the membrane in a direction opposite to the normal working filtering direction of the membrane, which may remove membrane fouling resulting from pore clogging to a certain extent. However, the gas, cleaning water or cleaning solution can not wash or soak the surface of the membrane that is in contact with the liquid to be filtered while the membrane is in its normal operation, where the gel layer and cake layer, as two major forms of membrane fouling, form and develop, thereby greatly degrading the filtering performance of the membrane. Conventional reverse hydraulic cleaning and reverse chemical cleaning can only act on the other surface opposite to the surface of the membrane that is in contact with the liquid to be filtered while the membrane is in its normal operation, and therefore can not achieve a good cleaning effect. Moreover, reverse hydraulic cleaning and reverse chemical cleaning generally use the membrane permeating liquid of the membrane separation device, i.e., the end product of the water treatment system, as the cleaning water or the solvent, which degrades the actual water producing capability of the membrane separation device to a certain extent, requires more membrane separation devices to meet a designated processing scale, and increases construction costs of the system.
Chinese patent applications 200580046369.5 and 200610011310.9 provide a cleaning method which uses concurrently forward hydraulic cleaning, reverse hydraulic cleaning, forward chemical cleaning and reverse chemical cleaning in a continuous period of time. This cleaning method can achieve a good membrane fouling removing effect. However, said period of time is generally long, during which the membrane separation device can not operate normally, and the water treatment plant is put in a standby or halt state. Meanwhile, reverse hydraulic cleaning or reverse chemical cleaning itself consumes a certain amount of the end product water. Therefore, if this cleaning method is applied frequently, the actual utilization of the membrane separation device and the water producing rate of the system will be greatly reduced, and more membrane separation devices have to be configured to meet a designated processing scale, thereby increasing construction costs of the system. And if this cleaning method is applied infrequently, then good filtering performance of the membrane separation device can not be maintained in a long term; and the duration of cleaning or the concentration of the chemical agent has to be extended or increased, otherwise, the filtering performance can not be thoroughly recovered. This cleaning method involves too many steps, is not well-planned and uses too much cleaning solution and chemical agent, thereby restricting its applications in actual water treatment.